Electrolytic recording paper



ie tts 3,616,338 ELEtZTROLYTlC RECGRDHN G PAPER Gerald Mandel, Flushing,N.Y., assignor, by mesne assignments, to Hogan Faxirniie Corporation, acorporation of Delaware No Drawing. Filed Aug. 4, 1958, Ser. No. 753,07513 tClaims. (Cl. 204-2) This invention relates to the art ofelectrically markable recording media and particularly concerns animproved electrolytic recording paper.

Recording paper of the electrolytic type is marked by disposing thepaper between electrodes consisting of a positive metal anode and anegative cathode. The paper is impregnated with an electrolytic solutioncontaining a marking compound. This compound is capable of reacting withions supplied by the anode to form a colored mark wherever the anodecontacts the paper.

Recording paper employed for facsimile recording purposes must meetcertain important requirements. The paper must have the ability toproduce marks having a density which is a function (preferably linear)of the current passed through the paper. The paper must remain activeand resist discoloration when stored for extended periods of time.Recorded marks should be stable when the paper is exposed to ambientconditions of light and air. The active ingredients in the impregnant ofthe paper must avoid discoloration of or diffusion to adjacent sheets ofmarked or unmarked paper of the same or different types.

It has been known heretofore that an electrolytically markable papercould be made with a marking compound that formed a dark complex ionwith the metal ions released by the anode. The marks thus formed havegenerally been described as colored lakes. While recording papers madewith such compounds are generally satisfactory it has been noted thatthey are subject to several disadvantages. There has been noted atendency to bleach of marked portions of paper when the paper is rolledor folded in a wet condition. A transfer effect has also been noted.This effect appears when dry paper in marked or unmarked statecontaining lake-forming marking compounds causes discoloration ofadjacent sheets of paper which do not contain any marking compound.

The present invention involves a high speed precipitation reaction toprovide a superior type of electrolytic recording paper. In this type ofpaper there is provided an impregnant containing a compound which formsa precipitant in aqueous solution as a reaction product with the metalions. 'In the present invention, the current functions only toelectrodeposit metal ions into the paper and no part of the ionizingelectric current is required to effect precipitation of the reactionproduct.

According to the present invention there is provided an electrolyticrecording paper which displays excellent stability in storage. Itproduces clearly defined marks which are stable and do not fade onstanding. The marks do not transfer or bleach. Unmarked paper in a moiststate retains its marking characteristics.

Recording papers embodying the present invention contain chromotropicacid as a marking agent or marking compound. This compound is also knownas 4,5-dihydroxy-2,7naphthalene-disulfonic acid disodium salt. Themarking compound forms a light colored yellow or green aqueous solutionand forms a dark colored insoluble compound with ions of a metal anodeso that the insoluble compound precipitates out of the solution.

Satisfactory results are obtained with metal anodes such as stainlesssteel containing iron which result in emerald green marks, or copperanodes which result in red-brown marks, or titanium anodes which producebrick red marks.

Chromotropic acid is a colorless crystalline compound which whollydissolves in an aqueous solution having a properly adjusted pH so thatupon impregnation of white porous paper with a solution which has beenrendered properly electroconductive there is obtained anelectrolytically markable recording paper. Upon application of asuitable metal anode to the impregnated paper and applying an electriccurrent, the metal electrodeposits into the impregnant in the paper anda colored precipitant is formed. A suitable electrolyte must be added tothe weakly acid impregnant to render it properly electrically conductivefor marking to occur.

It has been discovered that addition of chromotropic acid to aformulation containing tannic acid as the marking compound in anelectrolytic recording paper produces several desirable and unusualresults. Normally electrolytic recording paper containing tannic acidproduces marks having a purple tone or cast. If the marking current isincreased this purple color becomes most pronounced. In many recordingapplications it is desired that the resulting marks be black and freefrom any purple or other off-black tone. Addition of chromotropic acidto the formulation has the effect of preventing offblack marking toresult in dark black marks. It has also been noted that in tannic acidmarking papers it is necessary to increase the marking current toincrease the darkness of the recorded marks. Addition of chromotropicacid to the formulation results in production of darker and blackermarks without requiring an increase in the marking current.

Typical formulations of impregnants of paper which can be used are asfollows:

Chromotropic acid grams 5 Potassium nitrate do 10 Water cc Chromotropicacid 2 grams. Potassium nitrate 10 grams. Tannic acid 5 grams. Water e100 cc. Sulfuric acid (6 N) Suflicient to adjust the pH of the solutionfor optimum marking (about 1.25).

Chromotropic acid 2 grams. Potassium nitrate 10 grams. Tannic acid 5grams. Monosodium phosphate 0.1 gram, Sodium bitartrate 0.25 gram.Thiourea 0.15 gram. Water 100 cc. Sulfuric acid (6 N) Sufiicient toadjust the pH of the solution for optimum marking (about 1.25).

The potassium nitrate serves as an electrolyte to adjust the electricalconductivity of the solution. Other inorganic salts may be used for thispurpose such as NaCl, KCl, NaNO etc. The monosodium phosphate and sodiumbitartrate provide phosphate and tartrate ions which act to preventfringing or spreading of the recorded marks. Sources of citrate,oxalate, and maleate ions can be used for a similar purpose. Thioureaacts as a stabilizer to prevent discoloration of the impregnatedrecording paper in a marked and unmarked state.

Instead of sulphuric acid, hydrochloric acid, nitric acid, or otheracids may be used. Since the acid is an electrolyte the quantity of saltelectrolyte necessary will depend on the acid concentration. The pHadjustment selected, i.e., the quantity of acid used, will depend on thedepth of colored mark desired and type of metal anode used. A pH ofabout 1.25 has been found to result in satisfactory marking. The acidused must not adversely affect the precipitation reaction. If theimpregnant is made too strongly acid, the recorded marks tend to fadeand the porous paper support may be attacked and weakened by the acidcontent. If the impregnant is made too basic, the resulting marks assumea red or purple cast.

To prepare the recording paper with any of the substances mentionedabove, the marking compound and electrolyte are dissolved in water, oracid. White, porous paper having a high wet strength is then immersed inthe solution. Upon removal the paper is ready for being marked betweenelectrodes to which a direct current is applied between a cathode andanode applied in contact with opposite sides of the paper. The anodeelectrode must contain a metal, such as iron, which will electrodepositinto the impregnant during the application of electric current throughthe paper. The metal must be one which will form a dark coloredprecipitant with the marking compound.

For use in recording, the impregnated paper can be marked in a facsimilerecorder of the type described in Patent 2,575,959 issued to J. V. L.Hogan.

Certain specific combinations and compositions have been given by way ofexample. However, it will now be apparent to those skilled in the artthat various modifications and changes may be made within the spirit andscope of the invention as defined in the appended claims.

What is claimed is:

1. An electrolytic recording medium comprising an impregnated sheetcontaining as a marking compound chromotropic acid in anelectrolytically conducting solution.

2. An electrolytic recording medium according to claim 1 in which anadditional acid is incorporated therewith for pH adjustment.

3. An electrolytic recording medium according to claim 1 in which anacid solution is incorporated therewith, the acidity of the solutionbeingapproximately pH 1.25.

4. An electrolytic recording medium according to claim 1 in which anacid solution is incorporated containing at least one compound selectedfrom the group consisting of sulfuric acid, nitric acid, andhydrochloric acid.

5. An electrolytic recording-medium according to claim 1 in which astabilizer is incorporated to'prevent discoloration of the recordingmedium prior to marking and of the background thereof after marking.

6. An electrolytic recording medium according to claim 1 in which astabilizer is incorporated to prevent discoloration of the recordingmedium prior to marking and of background thereof after marking and anadditional acid solution is incorporated for pH adjustment.

7. An electrolytic recording medium according to claim 1 in whichthiourea is incorporated as a stabilizer.

8. An electrolytic recording medium according to claim 1 in which theelectrolyte is selected from the group consisting of alkali metalchlorides and alkali metal nitrates.

9. An electrolytic recording medium according to claim 1 in which theelectrolyte is selected from the group consisting of sodium andpotassium chlorides, and sodium and potassium nitrates.

10. An electrolytic recording medium according to claim 1 in which theelectrolyte is selected from the group consisting of alkali metalchlorides and alkali metal nitrates, and incorporated in a solutioncontaining at least one compound selected from the group consisting ofsulfuric acid, nitric acid, and hydrochloric acid.

11. An electrolytic recording medium according to claim 1 in which theelectrolyte is selected from the group consisting of nitrates andchlorides of sodium and potassium, and there is incorporated an acidsolution containing at least one compound selected from the groupconsisting of sulfuric acid, nitric acid and hydrochloric acid, andthiourea as a stabilizer. v

12. An electrolytic recording medium according to claim 1 in which isincluded a stabilizer to prevent discoloration of the recording mediumprior to marking and of the background thereof after marking and anadditional acid for pH adjustment, the impregnant consisting essentiallyin approximate parts by relative weights of chromotropic acid 2,electrolyte 10, acid 5 as said additional acid. and stabilizer 0.2 assaid stabilizer.

13. An electrolytic recording medium according to claim 1 in which thecomponents are in an aqueous solution and include a stabilizer toprevent discoloration of the recording medium prior to marking and ofthe background thereof after marking, the solution consistingessentially in approximate parts by relative weight of chromotropic acid2, sodium chloride 10, 6 N sulfuric acid 5, and thiourea 0.2 as saidstabilizer.

References Cited in the file of this patent UNITED STATES PATENTS2,358,839 Wagner Sept. 26, 1944 2,419,296 Solomon Apr. 22, 19472,433,632 Solomon Dec. 30, 1947 2,459,521 Grcig Jan. 18, 1949 2,485,678Tribble Oct. 25, 1949 2,541,488 Vanselow et al Feb. 13, 1951 2,776,252Schwartz Ian. 1, 1957 2,864,748 Mones Dec. 16, 1958 OTHER REFERENCESSpot Tests Inorganic Reactions, F. Feigl, Elsevier Pub. Co., N.Y. 1954,pages 187 and 188.

1. AN ELECTROYLTIC RECORDING MEDIUM COMPRISING AN IMPREGNATED SHEETCONTAINING AS A MARKING COMPOUND CHROMETROPIC ACID IN ANELECTROLYTICALLY CONDUCTING SOLUTION.
 12. AN ELECTROYLTIC RECORDINGMEDIUM ACCORDING TO CLAIM 1 IN WHICH IS INCLUDED A STABILIZER TO PREVENTDISCOLORATION OF THE RECORDING MEDIUM PRIOR TO MAKING AND OF THEBACKGROUND THEREOF AFTER MARKING AND AN ADDITIONAL ACID FOR PHADJUSTMENT, THE IMPREGNANT CONSISTING ESSENTIALLY IN APPROXIMATE PARTSBY RELATIVE WEIGHTS OF CHROMOTROPIC ACID 2, ELECTROLYTE 10, ACID 5 ASSAID ADDITIONAL ACID, AND STABILIZER 0.2 AS SAID STABILIZER.